Lasers and Spectroscopy

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Lasers and Spectroscopy

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Exciting Molecules

• Molecules can be excited using either broadband
  or monochromatic light. Spectra obtained using
  monochromatic light are easier to interpret. Why?
  In the UV/visible regions a broadband light
  source and monochromator can be used, but the
  intensity of light available to produce
  electronically excited states might be small.

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lasers are everywhere

• For spectroscopic experiments lasers are often
  the answer to providing light with the
  appropriate frequency and intensity.
• Lasers are also found in common devices such as
  DVDs and bar code scanners. We will consider
  initially the mechanism by which atomic lasers
  operate.

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Lasers and boltzmann

• Understanding lasers requires thinking about
  light, spectroscopy and Boltzmann. For a group of
  atoms or molecules (at thermal equilibrium at a
  given T) the Boltzmann expressions allow us to
  calculate the populations of the atomic and
  molecular energy levels if the energy level
  spacings and degeneracies are known!

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Boltzmann and Atoms

• For atoms, Boltzmann gives particularly simple
  expressions since there are no rotational or
  vibrational energies to consider. As well,
  electronic energy level spacings are so large
  that essentially all atoms are in the ground
  state (energy level) at ambient temperature.

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Atomic spectroscopy

• In atomic spectroscopy the movement of electrons
  between the ground and an excited state(s) is
  studied. Three mechanisms are important.
• 1. Stimulated absorption.
• 2. Spontaneous emission.
• 3. Stimulated emission.

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stimulated absorption

• Stimulated absorption occurs, for a two level
  system (E1 and E2) when a photon of frequency ?
  (E2 - E1)/h is absorbed.
• Before absorption After absorption
• E2
• h?
• E1

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spontaneous emission

• Electronically excited states are generally not
  stable. There is a high (well defined)
  probability that an electron in an excited state
  will revert (jump) back to the ground state over
  time. Atoms typically remain excited for short
  time periods (of the order of 10 ns). Process
  involves photon emission.

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spontaneous emission

• Spontaneous emission occurs, for a two level
  system (E1 and E2) when a photon of frequency ?
  (E2 - E1)/h is emitted.
• Before emission After emission
• E2
• 
  h?
• E1

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Stimulated emission

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stimulated emission

• In stimulated emission a photon of the correct
  frequency can cause an electron to move from the
  excited state to the ground state much more
  quickly than by the spontaneous emission route.
• Conservation of energy requires, of course, that
  two phtons are found after the stimulated
  emission step.

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stimulated emission

• Stimulated emission occurs, for a two level
  system (E1 and E2) when a photon of frequency ?
  (E2 - E1)/h is absorbed.
• Before absorption After absorption
• E2
• h?
  h?
• 
  h?
• E1

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Incoherent Radiation

• Spontaneous emission produces incoherent
  radiation. For a two level system all of the
  photons have the same frequency but the various
  photons produced have random phases and propagate
  in random directions (An incandescent light bulb
  is , in some respects, a similar example. Why?)

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Coherent radiation

• Stimulated emission produces coherent radiation -
  photons of the same frequency and phase moving in
  the same direction. In stimulated absorption the
  first (incident) photon is not absorbed. The
  two photons available after the stimulated
  emission can quickly cause other excited atoms to
  emit photons.

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stimulated emission - lasers

• Stimulated emission causes a chain reaction
  which produces the coherent and intense light
  beam seen in a laser.
• Stimulated emission is an extremely effective
  mechanism for depopulating excited states. A
  functioning laser requires an effective means of
  populating excited states.

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Class Handouts

• 1. Handout on physical setup and operating
  conditions of the He/Ne laser
• 2. Handout on energy levels of He and Ne and the
  allowed radiative and collisional transitions.
  Selection rules for atomic spectra are important.

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population inversion

• The efficiency with which electronically excited
  states can be produced gives rise to a
  population inversion for some of the Ne excited
  states. The population inversion is critical
  since (for the 633nm transition shown in the
  handout) a photon moving through the He/Ne
  discharge is very likely to be replicated by
  stimulated emission.

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population inversion

• In fact, photon replication by stimulated
  emission is more likely than photon absorption by
  the complementary/reverse
• Process. (the stimulated emission process is
  sometimes called a cloning process for photons.)

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Frequency selection

• In practice, light of a single wavelength is
  desirable. In some cases particular frequencies
  of light can be selected by varying the length of
  the laser cavity. Analogous to a pipe organ or
  the PIAB where the particle is a photon and
• ?/2 L/n where L is the length of the tube.

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To the Point?